1. Field of the Invention
This invention relates to a fluid displacement apparatus, and more particularly, to a scroll type refrigerant compressor with a variable displacement mechanism.
2. Description of the Prior Art
Compressors used in an automotive air conditioning system are typically driven by an automobile engine's power, which is transmitted to the compressor through an electromagnetic clutch. If the compressor is not provided with a variable displacement mechanism, and if the engine is rotating at a high rate, the compressor will be driven at a high rate as well and the operating capacity of the compressor may be larger than necessary. The electromagnetic clutch operates to ensure proper functioning of the compressor. However, under these conditions, the operation of the electromagnetic clutch can cause a large change in the load on the engine, thereby reducing the speed and acceleration performance of the automobile.
A solution to this problem is to provide the compressor with variable displacement mechanism. Scroll type compressors having variable displacement mechanisms for varying the compressor capacity are generally known in the art. Such a compressor is disclosed, for example, in U.S. Pat. No. 4,904,164 issued to Mabe et al.
With reference to FIG. 1, a scroll type compressor includes a housing 10 having a front end plate 11 and a cup-shaped casing 12. Front end plate 11 has an opening 111 through which drive shaft 13 passes. An annular projection 112 extends from a rear surface of front end plate 11. Annular projection 112 faces cup-shaped casing 12 and is concentric with opening 111. Annular projection 112 extends into cup-shaped casing 12, such that an outer peripheral surface of annular projection 112 is adjacent an inner wall surface of opening 121 of cup-shaped casing 12. Opening 121 of cup-shaped casing 12 is thus covered by front end plate 11. An O-ring 14 is placed between the outer peripheral surface of annular projection 112 and inner wall of opening 121 of cup-shaped casing 12 to seal the mating surfaces thereof. An annular sleeve 16 longitudinally projects forward from a front end surface of front end plate 11. Annular sleeve 16 surrounds a portion of drive shaft 13 and partially defines a shaft seal cavity 161. A shaft seal assembly 18 is coupled to drive shaft 13 within shaft seal cavity 161. Drive shaft 13 is supported by annular sleeve 16 through a bearing 17 located within the front end of sleeve 16. Drive shaft 13 has a disk-shaped rotor 131 at its rearward end. Disk-shaped rotor 131 is rotatably supported by front end plate 11 through a bearing 15 located within opening 111 of front end plate 11. A pulley 201 is rotatably supported by a bearing 19, which is disposed on the outer peripheral surface of annular sleeve 16. An electromagnetic coil 202 is fixed by a support plate about the outer surface of annular sleeve 16 and is disposed within pulley 201. An armature plate 203 is elastically supported on the forward end of drive shaft 13. Pulley 201, electromagnetic coil 202 and armature plate 203 form an electromagnetic clutch 20.
A fixed scroll 21, an orbiting scroll 22 and a rotation preventing/thrust bearing mechanism 24 for orbiting scroll 22 are disposed in the interior of housing 10. Fixed scroll 21 includes a circular end plate 211 and a spiral element 212 affixed to and extending from a forward end surface of circular end plate 211. Fixed scroll 21 is fixed within cup-shaped casing 12 by screws (not shown), which are screwed into circular end plate 211 from outside of cup-shaped casing 12. Circular end plate 211 divides the interior of housing 10 into a front chamber 27 and a rear chamber 28. Spiral element 212 of fixed scroll 21 is located within front chamber 27. A partition wall 122 longitudinally projects from the inner end surface of the rear end portion of cup-shaped casing 112 to divide rear chamber 28 into a discharge chamber 281 and an intermediate chamber 282. The forward surface of partition wall 122 contacts the rear end surface of circular end plate 211. Orbiting scroll 22, which is located in front chamber 27, includes a circular end plate 221 and a spiral element 222 extending from a rear end surface of circular end plate 221. Spiral element 222 of orbiting scroll 22 and spiral element 212 of fixed scroll 21 interfit at an angular offset of approximately 180 degrees and a predetermined radial offset to form a plurality of sealed spaces between spiral element 212 and 222. Orbiting scroll 22 is rotatably supported by bushing 23, which is eccentrically connected to the inner end of disk-shaped portion 131 through a radial needle bearing 30. While orbiting scroll 22 orbits, rotation thereof is prevented by rotation preventing/thrust bearing mechanism 24, which is placed between front end plate 11 and circular end plate 221 of orbiting scroll 22.
Compressor housing 10 is provided with an inlet port 31 and outlet port 32 for connecting the compressor to an external refrigeration circuit (not shown). Refrigeration fluid from the external refrigeration circuit is introduced into suction chamber 271 through inlet port 31, and flows into the plurality of sealed spaces formed between spiral elements 212 and 222. The plurality of sealed spaces between the spiral elements sequentially open and close during the orbital motion of orbiting scroll 22. When these spaces are open, fluid to be compressed flows into these spaces. When the spaces are closed, no additional fluid flows into these spaces and compression begins. The outer terminal end of spiral elements 212 and 222 terminate at a final involute angle, the location of the spaces is directly related to the final involute angle. Furthermore, refrigeration fluid in the sealed space is moved radially inwardly and is compressed by the orbital motion of orbiting scroll 22. Compressed refrigeration fluid at a central sealed space is discharged to discharge chamber 281 past valve plate 231 through discharge port 213 formed at the center of circular end plate 211.
A pair of holes (only one hole is shown as hole 214) are formed in circular end plate 211 of fixed scroll 21 and are symmetrically placed so that an axial end surface of spiral elements 222 of orbiting scroll 22 simultaneously crosses over both holes. Hole 214 (and the other holes not shown) provide fluid communication between the plurality of sealed spaces and intermediate pressure chamber 282. Hole 214 is placed at a position defined by involute angle (.phi.) (not shown) and opens along the inner side wall of spiral element 212. The other hole is placed at a position defined by involute angle (.phi..multidot..pi.) (not shown) and opens along a radially outside wall of spiral element 212. A pair of valve plates (only one valve plate is shown as valve plate 341) are attached by fasteners (not shown) to the end surface of circular end plate 211 opposite hole 214 and the other holes, respectively. Valve plate 341 and the other valve plate (not shown) are made of a material having spring constant which biases valve plates 341 against the openings of holes 214. When valve plates 341 are forced open due to a pressure difference between the pressure in front chamber 27 and rear chamber 28, valve retainer (not shown) prevents excessive bending of valve plates 341.
Circular end plate 211 of fixed scroll 21 also has communicating channel 29 formed therein and located at a radially outer side portion of the terminal end of spiral element 212. Communicating channel 29 provides fluid communication between suction chamber 271 and intermediate pressure chamber 282.
A control mechanism 36 controls fluid communication between suction chamber 271 and intermediate pressure chamber 282. Control mechanism 36 comprises a first valve element 37 having a cylinder 371 and a piston 372 slidably disposed within cylinder 371. Control mechanism 36 also comprises a second valve element 38. A first opening 373, which opens to intermediate pressure chamber 282, is formed through a side wall of cylinder 371. A second opening 374, which opens to communicating channel 29, is formed at a bottom portion of cylinder 371. A sealing ring member 61 is disposed on an inner surface 122a of partition wall 122. An axial annular projection 376 outwardly projects from the bottom of piston 372. A plurality of communicating holes 377 are formed in axial annular projection 376 to provide fluid communication between the interior of piston 372 and space 60. A bias spring 39 is disposed between a rear end surface of circular end plate 211 and the bottom portion of piston 372 to urge piston 372 rightwardly in FIG. 1. An opening formed in cup-shaped casing 12 is normally blocked by a plug 62.
A hollow portion 378 is formed on the inner surface of cover 379 of cylinder 371. Portion 378 exists even if top portion 375 of piston 372 contacts the inner surface of cover 379 of cylinder 371. This configuration allows discharge gas to pass into cylinder 371. An orifice tube 63 is disposed in the side wall of cylinder 371 to lead discharge gas to hollow portion 378 from discharge chamber 281.
Second valve element 38 comprises a bellows 381. A needle ball type valve 382 is attached to a rear end of bellows 381 by pin member 383, and is disposed within piston 372. The bottom of bellows 381 has a screw portion 384, which screws into an inner surface of axial annular projection 376. Screw portion 384 adjusts the initial condition of bellows 381. A valve seat 385 is formed on piston 372. A bias spring 386 is disposed within valve seat 385 and urges needle ball type valve 382 toward screw portion 384. In addition, a sealing member 71 is disposed on an outer peripheral wall of piston 372 to seal a gap between an inner peripheral surface of cylinder 371 and the outer peripheral wall of piston 372.
The operation of control mechanism 36 is as follows. When the compressor is in operation, and is driven in a condition in which the suction pressure is relatively high (i.e., the heat load is relatively great), the pressure of communicating channel 29 is greater than that of predetermined operation pressure of bellows 381, and bellows 381 contracts. As a result needle ball-type valve 382 moves forward to block valve seat 385. Therefore, discharge gas pressure led into cylinder 371 through orifice tube 63 fills hollow portion 378 to urge piston 372 toward circular end plate 211 against the restoring force of bias spring 39. If the heat load is high enough, piston 372 blocks first and second openings 373, 374, thereby preventing communication between suction chamber 271 and intermediate pressure chamber 282.
When the compressor is driven at a condition in which the suction pressure is relatively low (i.e., the heat load is relatively small), bellows 381 expands due to the reduced suction pressure gas which passes into the interior space of piston 372 from communicating channel 29 through communication holes 377. Therefore, needle ball-type valve 382 moves toward cover 379 to open valve seat 385. When valve seat 385 is opened, discharge gas led into hollow portion 378 through orifice tube 63 passes through valve seat 385 through the interior of piston 372, and through communicating holes 377 to communicating channel 29. Consequently, the pressure on the rearward side of top portion 375 is reduced and the rearward bias of piston 372, caused by the suction pressure and the restoring force of bias spring 39, overcomes the forward (leftward in FIG. 1) bias of piston 372. As first and second openings 373 and 374 are opened, communication between suction chamber 271 and intermediate pressure chamber 282 is restored.
Such an arrangement controls the capacity of the compressor by regulating the amount of the fluid flowing from the discharge chamber to the suction chamber via the intermediate chamber when the capacity of the compressor increases undesirably when the compressor is rotated at a high speed. However, such capacity control mechanism of the compressor has many parts and a complex construction. Therefore, this arrangement causes increased cost and assembly time during manufacture of the compressor. Other problems exist with prior art compressors as will be understood by those having ordinary skill in the pertinent art.